Standard Member

hi, I have a sony 1080 amp which i run wharfdale evo speakers with, I have just noticed the speakers are 6 ohm but had my amp set to 8 ohms ! its been like this for about 3 weeks, I have changed the amp down to 4 ohms. has this done any damage? I have not noticed anything wrong yet.

Member

Don't worry about it too much. These values are nominal at best, as the impedance is fequency dependant - an 8 ohm speaker may well have a range of 4-12 ohms or wider. A 6 ohm speaker will not be that much different. I've been running 6 ohm speakers on a 8 ohm for years.

Note: It's quite unusual for a modern amp even to have an output impedance setting. It suggests that the spec's on the output devices and power supply are not as generous as they might be.

Active Member

The speaker rating of 4 or 8 Ohm is a nominal rating or DC resistance. The actual impedance (AC resistance) changes based on the frequency.
Thus a speaker could e.g. have an impedance from 3.5 to 14.4 Ohm (Dynaudio Contour 1.3MkII) but it's a so-called 4 Ohm speaker.

Actually it's quite common that budget amps have this switch because the capabilities of the power supply are limited (small space, has to be cheap) - thus the manufacturer introduces the impedance switch to limit the current to the power amp stage (at 4 Ohm it's most likely only half compared to 8 Ohm). This prevents overheating and the protection circuit kicking in too often when the amp is driven too hard.

You probably won't notice any difference at normal volume levels (as only a few Watt of the maximum are used), but the 4 Ohm settings would limit the dynamics / headroom.
Using the 8 Ohm setting wrongly is unlikely to cause any damage since most amps have a protection circuit - just don't change the setting while the amp is on.
If you intentionally run it with 4 Ohm speakers in the 8 Ohm position make sure that the amp is not pushed too hard and overheats or distorts - in which case you should lower the volume immediately (or switch it off for cooling down).